The session expands on lengthy talks he held the day before with German chancellor Angela Merkel. The six leaders also discussed steps to resolve conflicts in Syria and eastern Ukraine, the White House said.

It said Mr Obama thanked his counterparts for their cooperation during his two terms in office.

He urged them to work on common challenges with a Trump administration on the "basis of the core values that define the United States and Europe as open democracies".

Since Mr Obama's arrival on Wednesday on his sixth and last trip to Germany as president, he and Mrs Merkel have focused several meetings on issues of globalisation and trans-Atlantic cooperation.

The talks come largely in the context of what the election of Mr Trump will mean to efforts to seek peace in Ukraine and Syria, the strength of the Nato alliance, trade agreements, efforts to fight climate change, and other pressing matters.

The White House said in a summary of the meeting that the leaders expressed grave concern about the humanitarian situation in Aleppo.

They agreed that increased attacks by the Syrian regime and its supporters, including Russia and Iran, should be immediately halted. They also are calling for humanitarian access to the city to be restored.

On Ukraine, the White House said the leaders agreed that sanctions against Russia must remain in place until it meets its commitments to resolving the conflict.

Mr Obama also briefed his counterparts on the progress made in Iraq in liberating Mosul from Islamic State militants.

The White House said the leaders agreed on the need for stability after the city is liberated. Mr Obama also encouraged leaders to continue efforts to expand information sharing throughout the European Union to help disrupt terror attacks.

At a news conference with Mr Obama on Thursday, Mrs Merkel diplomatically said she was approaching the incoming Trump administration "with an open mind".

Around the world, many are looking to Mrs Merkel - one of the longest serving leaders of a major world power, leader of Europe's largest economy and one of the biggest US trade partners - for leadership as Mr Obama leaves office.

Mr Obama seemed to underscore that reality as he effusively thanked Mrs Merkel for her "deep friendship".

He said he could not "ask for a steadier or more reliable partner on the world stage", adding that he would vote for her if she chose to run again.

Joining the two leaders were the heads of countries at the centre of many of the European Union's coming challenges.

Prime Minister Theresa May is preparing the UK for negotiations to leave the trade bloc. Spain's prime minister Mariano Rajoy faces economic woes in his country that have contributed to financial instability in the EU.

Italian PM Matteo Renzi's already shaky economy has been rocked by tens of thousands of refugees and French president Francois Hollande's socialist government faces a major challenge from the far-right in elections next year.

Mr Trump has applauded the British decision to exit the EU and has had meetings with Nigel Farage, leader of Ukip and a key player in the British decision to leave.

For his part, Mr Obama said his hope was that the Brexit negotiations be "conducted in a smooth and orderly and transparent fashion and preserve as closely as possible the economic and political and security relationships between the UK and EU".

Still, Obama noted that he considered the EU "one of the world's great political and economic achievements".